1. Field of the Invention
This invention relates generally to photographic recorders of the ray type and more particularly to mirror scanning systems compensating for various orientations of mirror facets by corresponding alterations in the direction of light rays emanating from an acousto-optic modulator.
2. Description of the Prior Art
Optical scanning systems are utilized in a variety of situations including non-impact printers and facsimile devices. Typically, such systems employ a multi-faceted mirror which is rotated about a stationary axis to provide for a scanning of a light beam within a plane, usually the horizontal plane. For the imprinting of alpha-numeric characters or other symbols on a recording medium, the medium may be formed of photo-sensitive material and positioned along the periphery of a drum. The axis of the drum lies in the scanning plane. The drum is rotated by a predetermined increment between successive scans so that each scan produces one line of printed material on the recording medium.
For modulating the intensity of the light beam so as to produce light and dark regions on the recording medium, many systems are now employing acousto-optic modulators wherein an interaction between an acoustic wave and a light wave results in a splitting off of a portion of the optical energy from the main beam, or zero order beam, into a diffracted, or first order beam. The scanning mirror and the drum are positioned to intercept the diffracted beam. Thus, by activating and deactivating the acoustic wave, the diffracted beam is amplitude modulated.
In the use of the foregoing modulators, the angle of diffraction, or Bragg angle, is dependent on the frequency of the acoustic wave and on the speed of propagation in the medium of the modulator as these two parameters determine the wavelength of the acoustic wave. Such frequencies are on the order of radio frequencies, RF, and may approach 100 MHz (megahertz). Accordingly, in the construction of such scanning systems, the locating of the optical components is dependent on the selection of the acoustic frequency and the Bragg angle so that the diffracted beam will be intercepted by the scanning mirror and reflected to the site of the scan line on the recording medium.
A limitation arises in the construction of such systems in that great precision is required to insure that a succession of scan lines will appear to be uniformly presented on the recording medium, and, in particular, that the scan lines will be evenly spaced. However, due to the geometry of the optical paths and the optical elements, a slight misalignment in the vertical orientation of a mirror facet relative to a second facet results in a noticeable variance in the spacing between lines of printed matter on the recording medium. Accordingly, it has been necessary to maintain high tolerance on the manufacture of the scanning mirror. It would be most advantageous if the scanning system would be less dependent on the precision of the manufacture of the scanning mirror for increased accuracy in the spacing of scan lines even in the presence of discrepancies in the orientations of the mirror facets.
This limitation is compounded by considerations which become apparent during an attempt to build the scanning system with features which might compensate for discrepancies in the facet orientations. For example, the use of fixed compensation, established individually for each mirror facet as by perturbations in the beam direction, requires a time consuming alignment process. Also, the use of an optical sensor for an automatic compensation for the facet errors introduces the considerations of electronic component drift which would introduce much greater errors than those of the facet orientations.
The foregoing illustrates limitations of the known prior art. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations as set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.